ORIENTATION OF ANTIGEN-BINDING SITES IN DIMERIC AND TRIMERIC SINGLE-CHAIN FV ANTIBODY FRAGMENTS

Electron microscopy of dimeric and trimeric single chain antibody Fv f ragments (scFvs) complexed with anti-idiotype Fab fragments was used t o reveal the orientation of antigen binding sites. This is the first s tructural analysis that discloses the multivalent binding orientation of scFv trimers (triabodies), Three different scFv molecules were used for the imaging analysis; NC10 scFv-5 and scFv-0, with five- and zero -residue linkers respectively between the V-H and V-L domains, were co mplexed with 3-2G12 anti-idiotype Fab fragments and 11-1G10 scFv-0 was completed with NC41 anti-idiotype Fab fragments. The scFv-5 molecules formed bivalent dimers (diabodies) and the zero-linker scFv-0 molecul es formed trivalent trimers (triabodies), The images of the NC10 diabo dy-Fab complex appear as boomerangs, not as a linear molecule, with a variable angle between the two Fab arms and the triabody-Fab completes appear as tripods. (C) 1998 Federation of European Biochemical Societ ies.